Ionic size effect on magnetic and electronic properties of La0.70A0.05Sr0.25CoO3 (A=La, Nd, Gd, Ho, Y)

The structural, magnetic and electronic transport properties of La0.70A0.05Sr0.25CoO3 (A=La, Nd, Gd, Ho, Y) have been investigated by X-ray diffraction and the magnetization and resistivity measurements. All the samples crystallize into hexagonal lattice with the space group R-3C (A167) at room temperature. Small doped ions shrink the lattices almost linearly and isotropically, and bend the Co-O-Co bond angle. Although the Co atoms in all compounds are stabilized in intermediate-spin electronic configuration, the (residual) resistivity (T→0) increase significantly with doping of the small ions. At low temperatures, the compounds change from metallic state of La0.70A0.05Sr0.25CoO3 (A=La, Nd, Gd) to insulated state of La0.70A0.05Sr0.25CoO3 (A=Ho, Y). The 5-7% magnetoresistance (MR) near Curie temperature can be interpreted in terms of the external field suppression of the spin fluctuation, but the relative large MR in the insulator state of La0.70A0.05Sr0.25CoO3 (A=Ho, Y) (∼20% up to 50-kOe field at 5 K) probably results from the field-induced transfer integral and the tunneling effect.